1. Field of Invention
The present invention relates to measurement of fluid flow and more particularly to the determination of the flux of a moving fluid.
2. Description of the Prior Art
Many practical fluid measurement devices involve determination of fluid flow properties in a conduit or channel of some type. Over the past 50 years or so various charged particle techniques have been proposed to measure or approximate in some degree such variables of fluid motion as velocity, mass flow or flux, flow direction, fluid pressure and temperature or density. Such flow parameters can be useful in determining the number or total mass of particles per unit time passing through a conduit.
An early teaching on the subject of fluid measurements through the use of charged particle techniques is provided by Meyer in U.S. Pat. No. 1,411,769 entitled Method Of And Apparatus For Measuring The Flow Of Fluids which issued on Apr. 4, 1922. The Meyer concept is based upon producing a gas discharge between a pair of electrodes located in a flow stream and relating changes in discharge characteristics, as measured by external electrical circuits, to changes in the properties of the flow field. While it is well known that gas discharge conditions will be influenced by the properties of the flow field, such as velocity and density, this type of approach lacks the sensitivity required to make direct velocity or mass flow determinations and can introduce major perturbations in the flow field upon which measurements are desired.
A variation of the Meyer concept which eliminates the major perturbing influence of the discharge through use of a radioactive source to produce ionization in the flow is disclosed by Blake in U.S. Pat. No. 1,808,709 entitled Apparatus For Metering Gaseous Fluids issued June 2, 1931. In the Blake apparatus a radioactive source which ionizes the flow stream is located upstream of an electrode which is placed in the flow and is electrically isolated from the metal walls of the flow conduit. By placing an electrical field of insufficient intensity to produce ionization in the gas between the electrode and conduit wall a drift field for the charged or ionized particles produced by the radioactive source is established. As ionized particles are swept downstream from the region of the radioactive source, under the action of the flow field, they are collected either on the conduit wall or the electrode. This collection of charges constitutes a current which is directly relatable to the characteristics of the flow of gas through the conduit. While this configuration offers significant improvements over the Meyer apparatus the Blake apparatus suffers from extremely low signal levels and potentially a nonlinear response to flow conditions because of the overall configuration and relationship between the source of ionization and the current collecting structure.
A more advanced flow sensing device is taught by Genin et al in U.S. Pat. No. 2,514,235 entitled System For Controlling The Charging Of Storage Batteries issued on July 4, 1950. The relevant teaching in this patent is the use of a gaseous ion source which produces a point source of ions thereby limiting the rather broad distribution (both in energy and spatial dependence) produced by the radioactive source employed by Blake. The gas is passed through a conduit having the point source of ions located on one side and a series of opposing collection electrodes. Under conditions of zero gas flow the maximum number of ions are collected on the collector positioned immediately opposite the ion source. As the gas flow transverse to the ion stream is increased the ions are deflected downstream and collected on adjacent electrodes. A further teaching of this patent is the determination of flow quantity by measuring this deflection of the ion beam produced by the flow of gas transverse thereto. While the ion collectors employed to determine deflection of the ion beam produced by the flow stream were not of a nature suitable for determining the quantative features of the ion distribution they were sufficient to make qualitative measurements relatable directly to flow quantity. Furthermore, while not taught by Genin et al the measurement of flow quantity made in this manner is directly relatable to mass flow.
A subsequent teaching indicative of the then still developing art in the field of fluid measurements is provided by Mellen in U.S. Pat. No. 2,611,268 entitled Velocity Meter Of Gas Flow issued Sept. 23, 1952. Mellen employs a radioactive source configuration similar to that of Blake but utilizes two sets of collectors, one upstream and one downstream, to detect the presence of ionization produced by the radioactive source. With this technique improved sensitivity to flow properties over that achieved by Blake is obtained. Further, flow direction in the conduit can also be ascertained.
The Mellen concept is modified in U.S. Pat. No. 2,861,452 entitled Nuclear Anemometer issued to W. A. Morgan on Nov. 25, 1958 which also teaches apparatus for measuring the velocity and direction of a gas flow. The apparatus includes a conduit having a centrally located radioactive material on axis to ionize gas passing through the conduit. Positioned upstream and downstream from the radioactive source are a first and a second cylindrical electrode having radially extending electric fields. The ions produced in the gas from the action of the radioactive source are collected by the cylindrical electrodes. Because of the action of the flow field, ions produced in the upstream region are deflected in the downstream direction. Therefore, the difference in current collected by the two cylindrical electrodes is indicative of the velocity and flow direction of the gas passing through the anemometer.
In contrast to the teachings of Mellen and Morgan, the first actual demonstation of the extremely high sensitivity that can be achieved through the use of ion deflection techniques was the work reported by W. Fucks in Gas Discharges Applied To Measurement, Appl. Sci. Res., Vol. 5, Section B, p. 167 (1955). In this work, both radioactive and corona source ionization were used to produce charged particles which were subsequently deflected across the flow stream to determine various flow characteristics such as flow stream velocity and velocity fluctuations. This work taught that under constant ion source current conditions, direct velocity determinations could be made. Other work reported by F. D. Werner et al in Investigation Of A Corona Discharge For Measurement In Air Flow, University of Minnesota Institute of Technology, Department of Aeronautical Engineering, Res. Rep. 84, (1952), clearly demonstrated through the use of a corona source and proper collector configuration that either positive or negative charged particle distributions which were deflectible in the flow stream could be detected with high resolution. As in the case of the Fucks work, Werner operated under constant source current conditions and thereby determined flow velocity. Neither of these works reported or appear to have recognized the importance of determining mass flow by using charged particle deflection techniques.
Some of the more sophisticated devices to evolve from this sequence of developing art are disclosed by Durbin in U.S. Pat. No. 3,470,741 entitled Mass Flow Meter Apparatus issued Oct. 7, 1969. The fundamental operation of this type hardware is based on the Erikson air blast method of determining ion mobility which was reported in Phys. Rev. 20 117 (1922) and is described further in Basic Processes Of Gaseous Electronics, L. B. Loeb, University of California Press, Los Angeles, Calif., 1961 pgs. 8-13. The Durbin apparatus provides an electric field transverse to the direction of flow of fluid through a duct. The field is of sufficient intensity to produce ionization at a selected corona source location. The deflection of current from the corona source produced as a result of the transverse fluid flow is used to determine mass flow through the duct. In one type embodiment a single electrode on one side of the duct is maintained at an electric potential sufficient to generate ions in the flowing fluid and a cooperating split electrode on the opposite side of the duct collects such ions. The electric field maintained between the single and split electrodes has a gradient across the duct and causes the ions to migrate transversely under the influence of this field while simultaneously moving axially due to the velocity of the fluid. When the fluid is not moving through the duct, the current to each of the split collectors is the same. However, when the fluid is in motion, a differential electric current occurs at the split collector and the current differential is correlated to the mass flow with straightforward instrumentation techniques. In a second type of embodiment, Durbin maintains a radially graduated electric field transverse to the direction of fluid flow between an ion transmitter such as a corona source on the longitudinal axis of a duct, and an ion receiving electrode which is wound along the inner wall of the duct. Then by producing ions having a known distribution with the transmitting electrode and locating the displacement in the direction of fluid flow for the median ion in the distribution, the mass flow of the fluid can be determined. The Durbin teaching requires maintaining an electric field gradient transverse to the direction of the gas flow, an ion source having a known distribution and means for measuring the net deflection due to the mass transfer of the fluid in the duct of the ions forming the distribution.
The last and most recently reported work is that of G. S. Castle and M. R. Sewell, IEEE Transactions On Industry Applications Jan./Feb. 1975, Vol. IA-11, No. 1, pgs. 119-124. In this flow sensor a radioactive source similar in concept to that of Blake, Mellen and Morgan is employed to produce a source of ionization in the flow stream. By applying an electric field transverse to the flow direction and using a split electrode configuration opposing the radioactive source location the deflection of ions produced by the action of the transverse flow field can be measured. While this apparatus in principle can be employed to measure either velocity or mass flow the signal output is only linear over a small flow range, a problem common to use of radioactive sources employed in the configurations considered, as noted previously.